This invention relates to an electronically controlled fluid suspension system for controlling the attitude of a vehicle.
When a vehicle immediately starts or accelerates, the phenomenon of a squat occurs, that is, the height of the rear end of the vehicle is quickly lowered. Conversely, at the time of sudden braking or stopping, a dive occurs, or specifically, the height of the front end is quickly lowered. The cornering vehicle rolls, that is, the outer-wheel side of the cornering vehicle is lowered. In order to control the inclination of the vehicle caused by such phenomena as squat, dive and roll, various suspension systems have been proposed. For example, in a system disclosed in Japanese published unexamined patent application No. 61-64512, front and rear-wheel side valves are controlled to open during the time period set corresponding to the degree of acceleration, so as to control the vehicle's attitude.
The prior electronically controlled fluid suspension system controls the vehicle's attitude in accordance with the degree of acceleration. Therefore, at the time of rapid acceleration or braking, even large variances in the vehicle's attitude can be controlled and the vehicle's attitude prior to the rapid acceleration can be maintained. However, this prior system cannot improve the road-adherence ability or cannot maintain the steering stability during acceleration, braking or cornering.
It is generally known that the wheels of a vehicle are provided respectively with a fluid spring. By supplying or releasing fluid to or from the fluid spring, the vehicle's attitude is electronically controlled. For example, in a system disclosed in Japanese published unexamined patent application No. 61-57415, when a target vehicle height is changed, the air is supplied to or released from the fluid spring through a piping having a large diameter for a specified time period, and then through a piping having a small diameter. The suspension system adjusts the vehicle height rapidly and prevents overshooting. For another example, Japanese published unexamined patent application No. 61-110407 discloses a suspension system where the fluid supply and release sides of a vehicle are provided respectively with a fluid spring, and a control valve selects the fluid spring to supply or release the air thereto or therefrom. A fluid supply control valve for the front wheels of the vehicle is installed between the control valve and a fluid supply tank for the front wheels. Separately, a fluid supply control valve for the rear wheels of the vehicle is provided between the control valve and a fluid supply tank for the rear wheels thereof. Thereby, a quantity of fluid supplied to the fluid springs of the front and rear wheels can be changed to avoid unbalanced vehicle height due to variances in load conditions on the vehicle.
In the prior art suspension systems, the vehicle's height is changed wheel by wheel sequentially. Problem is that changing the vehicle height of a wheel affects the other wheels, that is, changes the vehicle height thereof. If fluid simultaneously starts to be supplied to or released from each fluid spring of the four wheels, and the fluid supply or release is stopped sequentially from the wheel which reaches the target height, the fluid spring which first terminates the fluid supply or release is influenced by the fluid spring which air is still supplied to or released from. The target vehicle height of the wheel having the influenced fluid spring varies, requiring fluid supply or release again to regain the target vehicle height. Therefore, the control over the vehicle's attitude is a time consuming work.